For example, the structure such as the part of the tool or the housing thereof, the mechanical structure and the housing thereof, the internal combustion engine having parts that are technically movable, the electric motor, and the transformer, etc. as well as the elastic structure such as the surface or the sound absorbing wall of the auto body of the vehicle like the automobile, are generally subjected to vibration. An influence of a sound generated thereon is transmitted via the air as a medium. In particular, restriction on a vehicle exterior noise of automobiles is getting strict, and it is urgent to reduce the vehicle exterior noise (such as an engine noise, a tire noise, and a muffler noise) emitted from the automobile to neighborhoods.
In the future, if an internal combustion engine is switched only to an electric automobile, it naturally releases the engine noise of the internal combustion engine and the noise of the muffler emitting an exhaust gas. However, there is no possibility of being released from the tire noise (road noise) generated by a contact of a tire thereof with a road.
FIG. 5 is a diagram showing generation of the tire noise at present, including not only one which is directly generated by the contact of the tire with the road, but also one which comes outside by being reflected at a wheel housing. On the other hand, in terms of the wheel housing, it reflects not only the tire noise but also part of the engine noise and the exhaust noise and becomes a source of the vehicle exterior noise.
As a countermeasure against such noise, a patent document 1 discloses a structure wherein a foam is filled in a center pillar of an automobile or the like for the purpose of insulating a wind sound or the like and wherein it expands at a high expansion ratio.
For a fender liner for protecting a fender from an impact of a small stone or the like thrown up by a tire, a splash or an impact of muddy water or the like in running on a puddle or the like, a synthetic resin molded plate is generally used. However, the synthetic resin molded plate has low sound absorption capability and has low sound insulation capability, since it creates resonance. Thereby, the engine noise and the road noise are not sufficiently reduced. In addition, the synthetic resin molded plate changes a shock such as an impact of a small stone or the like and a splash or an impact of muddy water into a sound in a frequency range that human can easily hear, so that the fender liner using the synthetic resin has low soundproofing capability. Thus, there is known a fender liner that has a sound absorbing material of a non-woven cloth or the like stuck to a predetermined portion on a surface at a fender side of the fender line to improve soundproofing capability.
Then, a patent document 2 provides a fender line that can mitigate an impact sound of a small stone, earth and sand or the like thrown up by a tire when an automobile is running, a splash noise by a splash or an impact of muddy water or the like in running on a puddle or the like. It can stand a wind pressure even if it is attached to a fender on a front wheel side, since it has a sufficient stiffness. Moreover, even if attached water freezes and accretion of ice is generated, the ice is easily peeled.
In a patent document 3, it is required to increase a thickness of a sound absorbing material in order to increase sound absorption capability in the range not more than a medium frequency, since it is very hard to achieve high sound absorption capability over a wide range of frequencies and since, for example, a porous sound absorbing material has a sound absorption characteristic that is adapted to a high frequency range (about 4000 Hz or more). However, such an increase in the thickness may increase a volume of the sound absorbing material and also increase a weight thereof. Thereby, restriction arises in installation of a sound absorbing structure. In addition, a method for combining the porous sound absorbing material with other film material or other sound absorbing material is effective for changing a sound absorption profile of the porous sound absorbing material to improve sound absorption capability in the medium frequency range. However, it may also lower the sound absorption capability which was originally excellent in the high frequency range. Thus, a thin and lightweight sound absorbing structure, which has excellent sound absorption capability in a medium to high frequency range where sensitivity of human ears is high, is provided. The sound absorbing structure is composed of a composite film sound-absorbing material, which has a plate-like body having a plurality of apertures and a thin film disposed on the plate-like body and which is disposed at a sound source side, and a porous sound absorbing material disposed adjacent to the composite film sound-absorbing material. The thin film has a thickness of 2 μm to 50 μm and an elastic modulus of 1×106 to 5×109 Pa.